Transmission method

ABSTRACT

A transmission method in which a first signal frame, carrying a payload in a first part of the signal frame and second information in a second part of the signal frame, is transmitted intact between a signal source and a signal destination by copying the second information into the first part of at least one additional signal frame, transmitting both frames between the signal source and the signal destination and, at the destination, reconstructing the first signal frame by retrieving its original second information from the first part or parts of the additional signal frame or frames and placing the retrieved second information in the second part of the first signal frame. A typical application of the transmission method lies in its use in an SDH transmission system.

The invention relates to a transmission method.

The transmission method is especially suitable for transmitting a signalelement which includes first information in a first part of the signalelement and associated second information in a second part of the signalelement for the management of the signal element as it is moved betweena signal source and a signal destination, in a transmission system whichmay change at least some of the second information in the second part ofthe signal element as the signal element is moved between the signalsource and the signal destination.

A signal frame of a digital transmission system is a signal elementwhich includes information in a first part of the signal element andassociated second information in a second part of the transmissionelement for the management of the signal element as it is moved betweena signal source and a signal destination. In the transmission of asignal frame along signal stages between the signal source and thesignal destination, the associated second information is modified atnodes between the stages.

The invention provides a transmission method for a first signal elementwhich includes first information in a first part of the signal elementand associated second information in a second part of the signal elementfor the management of the signal element as it is moved between a signalsource and a signal destination, in a transmission system which maychange at least some of the second information in the second part of thesignal element as the signal element is moved between the signal sourceand the signal destination, including the steps of:

-   -   providing the second information also in the first part of at        least one additional signal element,    -   transmitting the first signal element and the additional signal        element or the additional signal elements from a signal source        to a signal destination and,    -   at the signal destination, providing a new signal element in        which the first information in its first part is the same as the        first information in the first part of the first signal element        and the information in its second part is the same as the second        information in the first part of the additional signal element        or elements, thereby providing the first signal element in        substantially its original form at the signal destination.

Examples of the second information include transmission managementinstructions, routing management instructions, performance managementinformation and configuration management information.

Preferably, the method includes the step of replacing the first signalelement by an identical replacement signal element prior to transmissionand using the replacement signal element in place of the first signalelement during and after transmission.

Preferably, a method for transmitting a plurality of signal elementswhich include information in respective first parts of the signalelements and common associated second information in respective secondparts of the signal elements for the management of the signal elementsas they are moved between the same signal source and signal destination,including the steps of:

-   -   providing the common associated second information as for the        plurality of signal elements in the first part of at least one        additional signal element and,    -   at the signal destination, providing a plurality of new signal        elements in which the first information in their respective        first parts is the same as the first information in the first        parts of the plurality of signal elements and the second        information in their respective second parts is the same as the        second information in the first part of the additional signal        element.

Preferably, the second information includes instructions for effectingthe alignment of the signal elements at the destination and the methodincludes the step of using the second instructions to align the signalelements of the plurality of signal elements at the destination.

Preferably, a signal element is a signal frame that is transmitted overa digital communications system.

Preferably, the information carried in the or each additional signalframe is error protected.

Preferably, the information carried in the or each additional signalframe is subject to forward error correction (FEC).

An SDH network capable of so operating as to perform the transmissionmethod on an SDH signal may be used to perform the method.

An SDH network capable of so operating as to perform the transmissionmethod on a SONET signal may be used to perform the method.

A SONET network capable of so operating as to perform the transmissionmethod on a SONET signal may be used to perform the method.

A SONET network capable of so operating as to perform the transmissionmethod on an SDH signal may be used to perform the method.

An appropriately arranged SDH network element included in an SDH networkprovides an SDH network capable of so operating as to perform thetransmission method on an SDH signal.

An appropriately arranged SDH network element included in an SDH networkprovides an SDH network capable of so operating as to perform thetransmission method on a SONET signal.

An appropriately arranged SONET network element included in a SONETnetwork provides a SONET network capable of so operating as to performthe transmission method on a SONET signal.

An appropriately arranged SONET network element included in a SONETnetwork provides a SONET network capable of so operating as to performthe transmission method on an SDH signal.

A method in accordance with the invention will now be described, by wayof example only, with reference to the accompanying drawings in which:

FIG. 1 represents a frame of a Synchronous Digital Hierarchy (SDH)network as defined in an International Telecommunications Union (ITU)document G.707,

FIG. 2 represents an SDH arrangement including first and second SDHnetwork elements capable of generating signals in accordance with theinvention connected to an SDH network,

FIG. 3 shows the generation of two SDH frames from a single SDH frame inaccordance with the invention,

FIG. 4 shows the reduction of the two SDH frames of FIG. 3 to a singleSDH frame in accordance with the invention,

FIG. 5 shows the separation of an SDH frame into three associated SDHframes in accordance with the invention,

FIG. 6 shows the separation of a plurality of SDH frames into associatedpairs of SDH frames in accordance with the invention and

FIG. 7 shows the provision of an additional SDH frame to a plurality ofSDH frames in accordance with the invention.

With reference to FIG. 1 of the accompanying drawings, an SDH STM-1frame 1 includes a first part 1 a consisting of a Path Overhead (POH)and a Virtual Container (VC) for carrying first information as an SDHpayload and a second part 1 b for carrying second information includingMultiplex Section Overhead (MSOH) and Regenerator Section Overhead(RSOH).

With reference to FIG. 2 of the accompanying drawings, an SDHarrangement includes an SDH network 4, a first SDH network element 3,capable of generating two frames from a single frame, connected to theinput side of the SDH network 4 and a second SDH network element 5,capable of generating a single frame from two frames, connected to theoutput side of the SDH network 4. The first SDH network element 3 has aninput port 2 and the second SDH network element 5 has an output port 6.

In the operation of the arrangement shown in FIG. 2, two SDH frames asshown in FIG. 3 are generated from an SDH signal frame that enters theinput port 2 of the first SDH network element 3, the two frames passthrough the SDH network 4 and the two frames reach the second SDHnetwork element 5, which reverses the process as shown in FIG. 4 of theaccompanying drawings.

With reference to FIG. 3 of the accompanying drawings, the signal frame1 which is the same as the signal frame shown at FIG. 1 above, givesrise to two SDH frames 7 and 8. The payload of the VC and the POH of theframe 7 are the same as the corresponding parameters of the frame 1. TheRSOH and MSOH may be, but need not be, the same as the correspondingparameters of the frame 1. A part or all of the payload of the VC of theframe 8 consists of RSOH and MSOH of the frame 1. The RSOH for the frame8 differs from the RSOH for the frame 1 because RSOH ispayload-dependent and the MSOH of the frame 7 may be, but need not be,the same as the MSOH of the frame 1. The RSOH and the MSOH of the frames1, 7 and 8 are generated in accordance with the provisions of the ITUdocument G.707 referred to above.

The generation of the two frames 7 and 8 from the frame 1 is carried outby the first SDH network element 3 of FIG. 2 and the first SDH networkelement 3 presents the two frames 7 and 8 to the input of the SDHnetwork 4.

The frames 7 and 8 are presented to the SDH network 4 instead of theframe 1. The SDH network 4 moves the frames 7 and 8 to the samedestination. The frames 7 and 8 are delivered to the second SDH networkelement 5 with their original payloads in their first parts 7 a and 8 a.It is, of course necessary to be able to distinguish the frame 7 fromthe frame 8 either by the sequence of their transmission or in someother way.

The second SDH network element 5 so operates as to provide the originalsignal frame 1 including its original payload in its first part 1 a andits original second information in its second part 1 b. The result iseffected by retrieving the payloads from the signal frames 7 and 8, thepayloads remaining unchanged as the signal frames 7 and 8 pass throughthe SDH network 4. The payload from the first part 7 a of the signalframe 7 is the same as the payload of the first part 1 a of the signalframe 1 and the payload of the first part 8 a of the signal frame 8 isthe same as the RSOH and MSOH of the second part 1 b of the signal frame1. The modified RSOH and MSOH of the second parts 7 b and 8 b of thesignal frames 7 and 8 are processed as appropriate.

The reconstructed signal frame 1 is, in effect, the same as the originalsignal frame 1 in all respects and is transmitted from the output port 6of the digital transmission system. FIG. 4 of the accompanying drawingsillustrates the provision of the original frame 1 from frames 7 and 8.

The second SDH network element 5 is capable of acting in the oppositesense, permitting the transmission of an SDH frame from the port 6,through the second SDH network element 5 to the SDH network 4. In thatcase, the second SDH network element 5 performs as did the first SDHnetwork element 3 to generate two frames from a single input frame.Further, the first SDH network element 3 then operates in reverse andprovides a single frame from the two frames generated by the second SDHnetwork element 5.

In the event that a single additional frame does not have the VCcapacity to carry the RSOH and MSOH, more than one additional frame maybe used as is shown in FIG. 5 of the accompanying drawings. As isindicated above, steps need to be taken in order to distinguish theframes from one another in order that the first and second informationpayloads are correctly identified.

FIG. 6 illustrates the transmission method applied to a plurality ofsignal frames, carrying differing second information, by the provisionof a pair of frames in place of each frame. One of the pair of framescarries a copy of the first information from the original frame as itsfirst information and the second of the pair of frames carries thesecond information from the original frame in its first part, that is,as its first information.

The above transmission method may also be applied to a plurality ofsignal frames, carrying common second information, being transportedbetween the same source and destination, by the provision of anadditional signal frame that carries the common second information ofthe plurality of signal frames in its first part. The procedure is thatthe original plurality of signal frames is reconstructed at thedestination by reintroducing the original common second information intothe second parts of the plurality of signal frames by extracting thatcommon information from the first part of the additional signal frame.That arrangement is represented by FIG. 7 of the accompanying drawings.

The second information can include instructions for effecting thealignment of the signal elements at the destination. In that case theinstructions for effecting the alignment of the signal elements isincluded in a further signal frame, that is, a further frame to thoseshown in FIG. 5 or FIG. 6, the second information being retrieved andused to align the signal elements of the plurality of signal elements atthe destination.

The above transmission method is especially applicable to a SynchronousDigital Hierarchy (SDH) network as defined by documents published by theInternational Telecommunications Union (ITU), including their documentG.707, in which a basic signal frame called a Synchronous TransportModule level 1 [STM-1] may be transmitted.

In relation to the above transmission method, an STM-1 is a single frameincluding a first part for carrying a payload and a second part forcarrying management information, as shown in FIG. 1, including, forexample, routing management information, transmission managementinformation, performance management information or configurationmanagement information or a combination of the types of managementinformation (RSOH, PTR, MSOH).

The first part of an STM-1 is called a Virtual Container-n (VC-n) andseveral standard capacities of VC-n are available as identified below:

VC type VC bandwidth VC payload VC-11   1664 kbits/s   1600 kbits/sVC-12   2240 kbits/s   2176 kbits/s VC-2   6848 kbits/s   6784 kbits/sVC-3   48960 kbits/s   48384 kbits/s VC-4  150336 kbits/s  149760kbits/s VC-4-4c  601344 kbits/s  599040 kbits/s VC-4-16c  2405376kbits/s  2396160 kbits/s VC-4-64c  9621504 kbits/s  9584640 kbits/sVC-4-256c 38486016 kbits/s 38338560 kbits/s

The Virtual Container types VC-n-Xc (VC-4-4c, VC-416c, VC-4-64c andVC-4-256c) are X Continuously concatenated VC-ns, which are amultiplicity of Virtual Containers associated one with another with theresult that their combined capacity can be used as a single containeracross which bit sequence integrity is maintained.

The second part of an STM-1 carries a section overhead which ismanagement information including routing and transmission managementinformation including Regenerator Section Overhead (RSOH) and MultiplexSection Overhead (MSOH). Also carried in the second part of an STM-1 areinformation elements identified as J1, B3, C2, G1, F2, H4, F3, K3 andN1.

The following is an example of the above transmission method applied toan STM-1. All of the parameters referred to below are defined in thedocument G.707:

-   (a) The original payload, a VC-4, is mapped into a new VC-4.-   (b) All of the section overhead [SOH] (ie RSOH and MSOH) is mapped    into three concatenated VC-12s. As an alternative they may be mapped    into four VC-11s or into any other plurality of VCs including VC-2s,    VC-3s or VC-4s. As is indicated above, a multiplicity of Virtual    Containers associated one with another with the result that their    combined capacity can be used as a single container across which bit    sequence integrity is maintained provide concatenated containers.-   (c) As an alternative to (b) above, more efficient bandwidth    utilisation is achieved by mapping some but not all of the section    overhead, for example, by not mapping the framing word and the NU    (not used) bytes, reducing the number of VCs required to transport    the RSOH and MSOH. When that is done, only two VC-12s or three    VC-11s are required for the section overhead.-   (d) The same frame sequence coding as that used in the H4 bit    position for virtual concatenation is used to align the VC-4 frame    of the originating STM-1 with the frames of the concatenated VCs,    the coding being placed in any unused byte in the path overhead, for    example, the K3 byte.

Alternatively, an eighth bit is generated by multi-framing any number ofbits in the path overhead to generate an eight-bit code for the VC-4frame sequencing. For example, bits 5 and 6 which are not used in the K3bit position are four-frame multi-framed to generate an eighth bit whichis treated like an H4 bit to produce the VC-4 frame sequence. The valueof the frame sequence is copied to one of the unused bytes in the SOH,for example, the byte with co-ordinates 1, 2, 6 as defined in G,707.

-   (e) A new signal label code, to indicate the type of payload being    transported, is inserted into the C2 bit position. For example, the    code 1110 0001 may be used.-   (f) The VC-4s and the SOH of the originating STM-1, which are    transported over the SDH network, may be error-protected by using in    band or out of band Forward Error Correction (FEC).

Other Synchronous Transport Modules which a Synchronous DigitalHierarchy is capable of accommodating include STM-4, STM-16, STM-64 andSTM-256 which include, respectively, a VC-4-4c, VC-4-16c, VC-4-64c andVC-4-256c which are identified above.

The transmission method is applicable to the other STMs by providingsecond information which is the same for the Virtual Containers in thepayload part of at least one additional signal frame and providingfurther signal frames in the payload parts of which are placed alignmentinstructions for and linked to the respective signal frames. At thedestination, the second information and the alignment instructions areretrieved from the payloads of the additional and further signal framesand reassembled into the originating STM.

The transmission method is applicable to the transmission over SDH ofsignal frames dedicated to digital transmission systems including otherforms of networks, for example, Synchronous Optical NETworks (SONET)meeting American standards (ANSI). In that case, the SONET SOH is mappedinto one or more VCs of appropriate size (one VC-11 or VC-12 or anyother VC) and the SONET Synchronous Payload Envelope (SPE) is mapped toa VC-3. The SONET SPE is the counterpart of the SDH payload.

Using the transmission method, an SDH signal or a SONET signal may betransmitted over an SDH network by suitable choices of SDH VC inaccordance with the respective amounts of first and second informationinvolved. Alternatively, using the transmission method, an SDH signal ora SONET signal may be transmitted over a SONET network by suitablechoices of SONET VC in accordance with the respective amounts of firstand second information involved.

FIGS. 1 to 7 which relate to SDH systems are equally applicable to SONETsystems by appropriate adjustment of the amounts of the first and secondinformation involved and terminology.

With reference to FIG. 2, the SDH network element 3 is as defined in anITU document G.783 for SDH compatibility but so modified, possibly bythe inclusion of additional hardware and software, as to perform thefunctions of separating a frame into two frames as shown in FIG. 3, forsignal flow in one direction, and reconstructing an original frame fromtwo frames as shown in FIG. 4, for signal flow in the oppositedirection. The same applies to the SDH network element 5 shown in FIG.2.

Equivalent SONET network elements corresponding to the SDH networkelements 3 and 5 are obtained by the appropriate adjustments to dealwith the differences between the standards involved.

1. A method of transmitting data, comprising the steps of: receiving thedata in an original signal frame suitable for transmission across asynchronous network, the original signal frame including a first partcontaining first information and a second part containing secondmanagement information; transmitting the data between a signal sourceand a signal destination in the synchronous network capable of changingthe second management information if the original signal frame weretransmitted; generating two intermediate frames from the original signalframe during data transmission over the synchronous network; containingall of the first information in a first part of one of the intermediateframes, all of the first information remaining unchanged during the datatransmission over the synchronous network; containing the secondmanagement information in a first part of the other of the intermediateframes, the second management information remaining unchanged during thedata transmission over the synchronous network; retrieving the unchangedfirst information from the first part of the one intermediate frame;retrieving the unchanged second management information from the firstpart of the other intermediate frame; and generating from the retrievedunchanged information a reconstructed signal frame that is identical tothe original signal frame.
 2. The method as claimed in claim 1, whereinthe second management information of the original signal frame comprisestransmission and routing instructions.
 3. The method as claimed in claim1, wherein the generating step is performed by generating an additionalintermediate frame including a first part for containing a part of thesecond management information when the other intermediate frame hasinsufficient capacity to contain all of the second managementinformation.
 4. The method as claimed in claim 1, and configuring theoriginal signal frame as a plurality of signal frames, and providingalignment instructions for aligning the plurality of signal frames. 5.The method as claimed in claim 1, and error protecting the data.
 6. Themethod as claimed in claim 5, and subjecting the data to forward errorcorrection (FEC).
 7. The method as claimed in claim 1, wherein thesynchronous network is a synchronous digital hierarchy (SDH) network,and wherein the original signal frame is an SDH signal frame.
 8. Themethod as claimed in claim 1, wherein the synchronous network is asynchronous digital hierarchy (SDH) network, and wherein the originalsignal frame is a synchronous optical network (SONET) signal frame. 9.The method as claimed in claim 1, wherein the synchronous network is asynchronous optical network (SONET), and wherein the original signalframe is a SONET signal frame.
 10. The method as claimed in claim 1,wherein the synchronous network is a synchronous optical network(SONET), and wherein the original signal frame is a synchronous digitalhierarchy (SDH) signal frame.
 11. The method as claimed in claim 7,wherein the generating and retrieving steps are performed by SDH networkelements.
 12. The method as claimed in claim 8, wherein the generatingand retrieving steps are performed by SDH network elements.
 13. Themethod as claimed in claim 9, wherein the generating and retrievingsteps are performed by SONET network elements.
 14. The method as claimedin claim 10, wherein the generating and retrieving steps are performedby SONET network elements.
 15. An arrangement for transmitting datacontained in an original signal frame suitable for transmission across asynchronous network, the original signal frame including a first partcontaining first information and a second part containing secondmanagement information, comprising: the synchronous network beingoperative for transmitting the data between a signal source and a signaldestination, the synchronous network being capable of changing thesecond management information if the original signal frame weretransmitted; a transmit network element operatively connected to thesignal source, for generating two intermediate frames from the originalsignal frame during data transmission over the synchronous network, oneof the intermediate frames including a first part containing all of thefirst information, the first information remaining unchanged during thedata transmission over the synchronous network, and the other of theintermediate frames including a first part containing the secondmanagement information that remains unchanged during the datatransmission over the synchronous network; and a receive network elementoperatively connected to the signal destination, for retrieving theunchanged first information from the first part of the one intermediateframe, for retrieving the unchanged second management information fromthe first part of the other intermediate frame, and for generating fromthe retrieved unchanged information a reconstructed signal frame that isidentical to the original signal frame.
 16. A transmit network elementfor transmitting data in an original signal frame suitable fortransmission across a synchronous network, the original signal frameincluding a first part containing first information and a second partcontaining second management information, the transmit network elementcomprising: an input of the transmit network element being operativelyconnected to a signal source in the synchronous network capable ofchanging the second management information if the original signal framewere transmitted; the transmit network element being operative forgenerating two intermediate frames from the original signal frame duringdata transmission over the synchronous network, one of the intermediateframes including a first part containing all of the first information,the first information remaining unchanged during the data transmissionover the synchronous network, and the other of the intermediate framesincluding a first part containing the second management information thatremains unchanged during the data transmission over the synchronousnetwork; and an output of the transmit network element for outputtingthe unchanged information from the two intermediate frames toward asignal destination of the synchronous network.
 17. A method oftransmitting data, comprising the steps of: receiving the data in anoriginal signal frame suitable for transmission across a synchronousnetwork, the original signal frame including a first part containingfirst information and a second part containing second managementinformation; transmitting the data between a signal source and a signaldestination in the synchronous network capable of changing the secondmanagement information if the original signal frame were transmitted;operatively connecting a transmit network element to the signal source;generating two intermediate frames from the original signal frame byoperation of the transmit network element during data transmission overthe synchronous network, one of the intermediate frames including afirst part containing all of the first information, the firstinformation remaining unchanged during the data transmission over thesynchronous network, and the other of the intermediate frames includinga first part containing the second management information that remainsunchanged during the data transmission over the synchronous network; andoutputting the unchanged information from the two intermediate framesfrom the transmit network element toward the signal destination of thesynchronous network.
 18. A receive network element for receiving data ina reconstructed signal frame that is identical to an original signalframe suitable for transmission across a synchronous network, theoriginal signal frame including a first part containing firstinformation and a second part containing second management information,the receive network element comprising: an input of the receive networkelement for receiving two intermediate frames from the original signalframe from a signal source of the synchronous network capable ofchanging the second management information if the original signal framewere transmitted, one of the intermediate frames including a first partcontaining all of the first information that remains unchanged duringdata reception over the synchronous network, and the other of theintermediate frames including a first part containing the secondmanagement information that remains unchanged during the data receptionover the synchronous network; and the receive network element beingoperative for retrieving the unchanged first information from the firstpart of the one intermediate frame, for retrieving the unchanged secondmanagement information from the first part of the other intermediateframe, and for generating the reconstructed signal frame from theretrieved unchanged information.
 19. The receive network element asclaimed in claim 18, and comprising an output of the receive networkelement being operatively connected to the signal destination of thesynchronous network for outputting the reconstructed signal frame towardthe signal destination of the synchronous network.
 20. A method ofreceiving data in a reconstructed signal frame that is identical to anoriginal signal frame suitable for transmission across a synchronousnetwork, the method comprising the steps of: receiving the data in theoriginal signal frame with a first part containing first information anda second part containing second management information; transmitting thedata between a signal source and a signal destination in the synchronousnetwork capable of changing the second management information if theoriginal signal frame were transmitted; operatively connecting a receivenetwork element to the signal destination; receiving two intermediateframes from the original signal frame from the signal source during datareception over the synchronous network, one of the intermediate framesincluding a first part containing all of the first information, thefirst information remaining unchanged during the data reception over thesynchronous network, and the other of the intermediate frames includinga first part containing the second management information that remainsunchanged during the data reception over the synchronous network;retrieving the unchanged first information from the first part of theone intermediate frame; retrieving the unchanged second managementinformation from the first part of the other intermediate frame; andgenerating the reconstructed signal frame from the retrieved unchangedinformation, by operation of the receive network element.
 21. The methodas claimed in claim 20, and outputting the reconstructed signal framefrom the receive network element toward the signal destination of thesynchronous network.